One application of a multiple-view directional display is as a ‘dual-view display’, which can simultaneously display two or more different images, with each image being visible only in a specific direction—so an observer viewing the display device from one direction will see one image whereas an observer viewing the display device from another, different direction will see a different image. A display that can show different images to two or more users provides a considerable saving in space and cost compared with use of two or more separate displays.
A further application of a multiple view directional display is in producing a three-dimensional image. In normal vision, the two eyes of a human perceive views of the world from different perspectives, owing to their different location within the head. These two perspectives are then used by the brain to assess the distance to the various objects in a scene. In order to build a display which will effectively display a three dimensional image, it is necessary to re-create this situation and supply a so-called ‘stereoscopic pair’ of images, one image to each eye of the observer.
Three dimensional displays are classified into two types depending on the method used to supply the different views to the eyes;
#Stereoscopic displays typically display both of the images over a wide viewing area. However, each of the views is encoded, for instance by colour, polarisation state or time of display, so that a filter system of glasses worn by the observer can separate the views and will only let each eye see the view that is intended for it.
#Autostereoscopic displays require no viewing aids to be worn by the observer. Instead, the two views are only visible from defined regions of space. The region of space in which an image is visible across the whole of the display active area is termed a ‘viewing region’. If the observer is situated such that the left eye is in the left image viewing region and the right eye is in the right image viewing region, then a correct set of views will be seen and a three-dimensional image will be perceived.
For flat panel autostereoscopic displays, the formation of the viewing regions is typically due to a combination of the pixel structure of the display unit and an optical element, generically termed a parallax optic. An example of such an optic is a parallax barrier, which is a screen with vertical transmissive slits separated by opaque regions. This screen can be set in front of a spatial light modulator (SLM) with a two-dimensional array of pixel apertures as shown in FIG. 1. The pitch of the slits in the parallax barrier is chosen to be close to an integer multiple of the pixel pitch of the SLM so that groups of columns of pixels are associated with a specific slit of the parallax barrier. FIG. 1 shows an SLM in which two pixel columns are associated with each slit of the parallax barrier.
The display shown in FIG. 1 comprises an SLM in the form of a liquid crystal device (LCD) having an active matrix thin film transistor (TFT) substrate 1 and a counter-substrate 2, between which are disposed a liquid crystal layer forming a picture element (pixel) plane 3 with associated electrodes and alignment layers (not shown) as appropriate. Viewing angle enhancement films 4 and polarisers 5 are provided on the outer surfaces of the substrates 1 and 2 and illumination 6 is supplied from a backlight (not shown). A parallax barrier comprises a substrate 7 with a barrier aperture array 8 formed on its surface adjacent the LCD and an anti-reflection (AR) coating 9 formed on the other surface thereof.
The pixels of the LCD are arranged as rows and columns with the pixel pitch in the row or horizontal direction being p. The aperture array 8 comprises vertical transmissive slits with a slit width of 2 w and a horizontal pitch b. The plane of the barrier aperture array 8 is spaced from the pixel plane 3 by a distance s.
In use, two interlaced images—a left eye image and a right eye image in the case of an autostereoscopic 3-D display—are displayed on the pixel plane 3 of the SLM. The display forms a left viewing window 10 in which the left eye image is visible and a right viewing region 11 in which the right eye image is visible in a window plane at the desired viewing distance of the display. The window plane is spaced from the plane of the aperture array 8 by a distance ro. The windows 10 and 11 are contiguous in the window plane and have a width and pitch e corresponding to the average human eye separation. The half angle to the centre of each window 10, 11 from the display normal is illustrated at alpha.
FIG. 2 of the accompanying drawings shows the angular zones of light created from an SLM 12 and parallax barrier 13 where the parallax barrier has a pitch of an exact integer multiple of the pixel column pitch. In this case, the angular zones coming from different locations across the display panel surface intermix and a pure zone of view for image 1 or image 2 does not exist. In order to address this, for example for a front parallax optic, the pitch of the parallax optic is reduced slightly so that the angular zones converge at a pre-defined plane (termed the ‘window plane’) in front of the display. This change in the parallax optic pitch is termed ‘viewpoint correction’ and the effect is illustrated in FIG. 3 of the accompanying drawings. The viewing regions, when created in this way, are roughly kite shaped in plan view.
FIG. 4 of the accompanying drawings illustrates another known type of directional display in the form of a rear parallax barrier display. This is generally similar to the display of FIG. 1 except that in the front parallax barrier display shown in FIG. 1 the parallax barrier is disposed between the SLM and the viewing windows 10 and 11 whereas, in the rear parallax barrier display shown in FIG. 4, the SLM is disposed between the parallax barrier and the viewing windows 10 and 11.
FIGS. 1 and 4 describe an autostereoscopic display. A dual view display operates in an identical manner, but the angle of separation between the different images is larger. Instead of the two images being sent to left and right eyes (approximately 6.2 cm apart), images are sent to left and right people (separated by, for example, one metre). The two images displayed on the pixel plane of the SLM are not the left eye image and the right eye image of a stereoscopic image pair, but are two independent images. Where a dual view display is installed in a motor vehicle, for example, one image may be sent to the driver and another image may be sent to a passenger in the front passenger seat. The driver may see a road map, whereas the front seat passenger may see a film.
A multiple view directional display may display more than two images. To use the above example of a display installed in a motor vehicle, a display may be arranged to send a further view to a passenger in the back seat. The term ‘dual view display’ as used herein is not limited to a display that displays two independent views in two different directions, but also includes a directional display that display three (or more) independent images in three (or more) different directions.
FIG. 5a is a schematic plan view showing a dual view display installed in a motor vehicle. The display is displaying one image to the driver 15 of the vehicle and is displaying a second image to a front seat passenger 16. The regions 15a,16a outlined in broken lines indicate the viewing regions for the driver's image and the passenger's image respectively. A dual view display in a motor vehicle is generally installed in the vehicle's dashboard, so that the driver 15 and front seat passenger 16 both view the display at a direction of approximately 40° to the normal of the display. Reference 17 denotes an ‘image mixing region’ in which both the driver's image and the passenger's image are visible. The image mixing region is centred about the normal to the display 14, and an observer located in the image mixing region 17 will perceive cross-talk.
Trace (a) in FIG. 5b shows how the intensity of a typical liquid crystal display panel varies as a function of the lateral position of the observer relative to the display.
It can be seen that the LCD panel is optimised for viewing in the direction normal to the display face of the panel (referred to as ‘on axis’). At viewing angles greater than approximately ±20° to the normal of the display the intensity decreases significantly.
When the LCD panel is viewed from an angle of ±40° the intensity is reduced by almost 50% compared to the on-axis intensity. When a conventional LCD panel is used in the dual view display 14 of FIG. 5a, therefore, the driver 15 and passenger 16 will see an image that has a low intensity.
Furthermore, at a viewing angle of ±40° the intensity of the conventional LCD panel varies steeply with changes in the viewing angle. Thus, if the driver 15 or passenger 16 in FIG. 5a should move their head sideways the intensity of the image they see will vary, and this is irritating and could possibly cause discomfort.
Trace (b) in FIG. 5b shows an ideal intensity profile for the dual view display 14 of FIG. 5a. The intensity would ideally have maxima at viewing angles of ±40° to the normal to the display face and, moreover, the variation of intensity with viewing angle would ideally be small at viewing angles of ±40°. The intensity of the display would ideally be low for viewing angles near 0° (i.e. for angles near the normal to the display face), since light emitting at angles near 0° is wasted. Furthermore viewing directions that are along, or close to, the normal to the display face are in the image mixing region 17, and making the intensity of the display low for viewing angles near 0° means that an observer positioned in the image mixing region 17 will not experience cross-talk.